Searchlight Equipment

Searchlights were typically of three sizes - 90 cm, 120 cm and 150 cm. Searchlights worked on the principal of reflectors concentrating a very powerful light into a concentrated beam. The light source was usually an electric arc, the flame of which consisted of the vapor of carbon heated into incandescence at the temperature of 5,400 degrees F. As the positive carbon pencil in an electric arc tends to burn unevenly, it is rotated in the searchlight by a small electric motor, while an electrically driven fan kept the lamp clear of fumes.

  Above left: MV 150 cm searchlight. Right searchlight in action with a
  Sound Locator

Before the introduction of Searchlight Control Radar (SLC), Sound Locators were used to direct the beams on to the target. The equipment consisted of Sound Collectors which could be traversed through 360 degrees and a sight to allow for the time lag of the travel of sound. The Sound Collectors were traversed until they faced the source of the sound. Due to the low velocity of the travel of sound (approx 1,100 feet per second), the aircraft would no longer be at the source of the sound but will have moved on along its course (a product of its speed and the time taken for sound to travel form the aircraft to the locator). The sight allowed the ‘present’ location of the aircraft to be determined by allowing for the time lag. The weakness of the system lay in estimating an aircrafts speed. In Mk III Locators, the beam had to be directed by voice control but in Mk VIII and IX Locators there was an automatic course-finding sight and the movement of the projector was positively controlled by electrical means from the Sound Locator. Enemy aircraft would also sometimes approach targets with short glides to avoid detection by Sound Locators.

Right: MV Sound Locator

Searchlight Control Radar (SLC) was in the process of being developed during 1940 but results were not great at first. Further development and refinement saw the deployment of SLC from late 1941 onwards, with priority allocation given to the new Killer belts, followed by the Indicator belts and GDA zones.

  Above: SLC on a 150 cm projector. Right - SLC on a
  90 cm projector

Sighting of SLC was paramount to ensure the ‘time base picture’ on the screen was as clear as possible so that aircraft could be detected as they came within range. ‘Standing Echoes’ could be caused by barrage balloons and high ground within seven miles of the set and could be dense enough to mask an echo caused by an aircraft.‘Standing Echoes’ could be screened naturally, e.g. by ground, quarry walls, thick woods and buildings if they were within one mile of the set. Any natural screen had to cover from view objects liable to cause echoes at an eye level height of 15 ft, and also giving the set a clear view above 15 degrees with eye level height at 5 ft. Sets were not affected by wireless sets but the following should not be less than 50 yards from the equipment: Nissan huts, pylons, power cables, metal buildings, railway metals, electric railways and telephone wires.

 Above: sketch showing the requirements for screening SLC

References:
Contribution To Victory, MV Electrical Co, 1947
Britains Wonderful Fighting Forces, Odhams Press Ltd,
5th AA Brigade papers, TNA
Manual of Anti-Aircraft Defence, Vol II, Pamphlet No 1, WO, 1940

150cmprojector
searchlight
soundlocatormv
ELSIEsearchlightradar1942IWMH28386
slc on 150cm proj
SLC screen

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